tccgen: Remove volatile qualifier from function return types

Modify post_type() such that the volatile qualifier is
removed from function return types, as required by ISO C.
Previously, only the const qualifier was removed.

Added tests for all three standard qualifiers, although
the new test for restrict only passes because TCC ignores
that keyword.

tccgen.c

@@ -5119,9 +5119,9 @@ static int post_type(CType *type, AttributeDef *ad, int storage, int td)
             /* if no parameters, then old type prototype */
             l = FUNC_OLD;
         skip(')');
-        /* NOTE: const is ignored in returned type as it has a special
-           meaning in gcc / C++ */
-        type->t &= ~VT_CONSTANT; 
+        /* A function return value has the unqualified version of its
+           declared type. */
+        type->t &= ~(VT_CONSTANT | VT_VOLATILE);
         /* some ancient pre-K&R C allows a function to return an array
            and the array brackets to be put after the arguments, such 
            that "int c()[]" means something like "int[] c()" */

tests/tests2/150_return_qualifiers.c

@@ -0,0 +1,30 @@
+const int const_return(void);
+volatile int volatile_return(void);
+int *restrict restrict_return(void);
+
+_Static_assert(__builtin_types_compatible_p(__typeof__(const_return()) *, int *),
+               "const is removed from a return type");
+_Static_assert(__builtin_types_compatible_p(__typeof__(volatile_return()) *, int *),
+               "volatile is removed from a return type");
+_Static_assert(__builtin_types_compatible_p(__typeof__(restrict_return()) *, int **),
+               "restrict is removed from a return type");
+
+int const_return(void)
+{
+   return 0;
+}
+
+int volatile_return(void)
+{
+   return 0;
+}
+
+int *restrict_return(void)
+{
+   return 0;
+}
+
+int main(void)
+{
+   return 0;
+}